Field of the Invention
The present invention relates to an organic light emitting diode (OLED) display and a driving method thereof.
Discussion of the Related Art
An organic light emitting diode (OLED) display includes an OLED which emits light by itself and has advantages of rapid response speed, high light-emitting efficiency and brightness, and a wide field of view. Typically, an OLED has a structure illustrated in FIG. 1. The OLED includes an anode electrode, a cathode electrode, and organic compound layers formed therebetween. The organic compound layers typically include a Hole Injection Layer (HIL), a Hole Transport Layer (HTL), an Emission Layer (EML), an Electron Transport Layer (ETL), and an Electron Injection Layer (EIL). When a driving voltage is applied to the anode electrode and the cathode electrode, a hole passing through the HIL and an electron passing through the ETL move to the EML to form an exciton, and thus, the EML emits a visible light.
Such an OLED display has a plurality of pixels in a matrix, each including an OLED, and adjusts the brightness of the pixels according to the gray scale of video data. Each pixel includes at least a driving Thin Film Transistor (TFT) for controlling a driving current flowing in an OLED, a storage capacitor for controlling a gate-source voltage of the driving TFT at a constant level in one frame, and a switching TFT for programming the gate-source voltage of the TFT in response to a gate signal. The driving current is determined by the gate-source voltage of the driving TFT, which is based on the data voltage, and the brightness of each pixel is in proportion to the driving current flowing in the OLED.
In such an OLED display, the driving TFTs of the pixels may have different threshold voltages due to a process deviation or a gate-bias stress which may occur with the elapse of driving time. To address this problem, an OLED display may have a pixel structure for sampling a variation in the threshold voltages of the driving TFTs and for reducing or preventing the variation from affecting the driving current. For an existing OLED display, a sampling period is provided to sample the threshold voltage of a driving TFT before a data voltage is charged in a pixel so as to compensate for a variation in threshold voltages. As display panels with high resolutions are widely used, one horizontal period has become shorter and thus, the sampling period has become reduced. However, such a reduced sampling period may adversely affect the capability of compensating for a variation in the threshold voltages of driving TFTs as well as the quality of images displayed on the screen.